Acid–base Equilibria

Question

Which of the following aqueous salts gives a solution with pH ≈ 7 at 25°C?

RankGuru · Accepted Answer

1. **Salt Hydrolysis Rules**:
   - **Strong Acid + Strong Base** (e.g., $NaCl$): No hydrolysis, $pH = 7$.
   - **Strong Acid + Weak Base** (e.g., $NH_4Cl$): Cationic hydrolysis, $pH < 7$ (Acidic).
   - **Weak Acid + Strong Base** (e.g., $CH_3COONa$): Anionic hydrolysis, $pH > 7$ (Basic).
   - **Weak Acid + Weak Base** (e.g., $NH_4CH_3COO$): Both ionize.
2. **Case of Ammonium Acetate**: $pH = 7 + \frac{1}{2}(pK_a - pK_b)$.
3. **Values**: For acetic acid, $pK_a \approx 4.74$. For ammonia, $pK_b \approx 4.75$. 
4. **Outcome**: Since $pK_a \approx pK_b$, the acidic and basic tendencies of the ions cancel each other out.
5. **Conclusion**: **Ammonium Acetate ($NH_4CH_3COO$)** gives an approximately neutral solution.

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Acid–base Equilibria

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